docs: def on_subscribe(self):
          # type: () -> Callable
          """Decorate a callback function to handle subscritions.
  
          **Usage:**::
  
              @mqtt.on_subscribe()
              def handle_subscribe(client, userdata, mid, granted_qos):
                  print('Subscription id {} granted with qos {}.'
                        .format(mid, granted_qos))
          """
          def decorator(handler):
              # type: (Callable) -> Callable
              self.client.on_subscribe = handler
              return handler
  
          return decorator